1. Field of the Invention
This invention relates to a method for determining the concentration of a substance, and more particularly relates to a method for determining the concentration of a substance in a gaseous medium with a chemical sensor that has a nominal measuring range for the concentration of the substance.
2. Description of the Related Art
Chemical or electrochemical sensors for determining the concentration of substances in gaseous media have a nominal measuring range in which measurement is possible with a certain accuracy. If measurement is attempted outside of this range the sensor will operate inaccurately. If extreme deviations from the nominal measuring range are encountered, the sensor may also be damaged.
Sensors of this type are therefore suitable for such concentration measurements in gaseous media only when the anticipated concentration of the substance to be measured is expected to fall within the nominal measuring range. For a number of applications the nominal measuring range of the sensor makes it impossible to directly measure the concentration of the gaseous media since at least a portion of the anticipated concentration values exceeds that of the nominal measuring range of the sensor.
One known way to overcome this problem is to feed a measured dilution gas stream to the chemical sensor to thereby dilute the concentration of the substance in question to within the nominal measuring range of the sensor. Thus the actual value of the concentration of the substance in the gaseous medium can be determined by calculation from the meter reading using the dilution factor.
Specifically, therefore, a sample gas stream is first removed from the gaseous medium and mixed in a mixing device with a diluting gas stream, especially a stream of air obtained from the environment, to form a measured gas stream. The dilution ratio can be modified by varying the amounts of sample gas stream and diluting gas stream. The measured gas stream thus obtained is fed to the chemical sensor, which generates a detection signal as a measure of the concentration of the substance. As soon as the detected concentration falls outside the nominal measuring range, the concentration in the sample gas stream is corrected by adjusting the dilution ratio. The concentration of the substance in the gaseous medium is then calculated by evaluating the detection signal, taking the dilution ratio into account.
In this connection, knowledge, as exact as possible, of the dilution ratio takes on particular significance, since it directly determines the accuracy of the measurement result. For this purpose, the concentration of an (additional) substance component is measured before and after dilution and the dilution ratio is determined from this. This requires that this (additional) substance component be known and constant in one of the two positions, in other words before or after mixing. One possible solution referred to as being preferable consists in using an air-gas mixture as the diluting gas stream that is drawn from the environment. For example, oxygen can be used as a substance component that is known and is available in constant quantity before mixing with the sample gas stream. The dilution ratio can be determined reliably from the ratios of the oxygen concentration before and after mixing.
One disadvantage to this solution is that it is a relatively expensive way to determine the dilution ratio because it is necessary to provide an additional sensor, especially to evaluate its detection signal. The software expense involved is also not inconsiderable, especially when the dilution ratio must be adjusted in the event of dynamic processes, for example fluctuations in the concentration of the substance to be measured.
Accordingly, it would be advantageous to improve a method and a device for determining the concentration of a substance in a gaseous medium which can achieve high accuracy at a comparatively low cost. In particular, the nominal measuring range of the chemical sensor, even with concentration values of the substance of the gaseous medium that fall outside this nominal range, must be maintained with the simplest possible design means and with the lowest possible software expense.